Why android studio tells me that ErrorInByte.valueOf(n) is always true? I tested it and for

ErrorInByte.valueOf(326)

it equals null.

Warning message :

This inspection analyzes method control and data flow to report possible conditions that are always true or false, expressions whose value is statically proven to be constant, and situations that can lead to nullability contract violations.

Variables, method parameters and return values marked as @Nullable or @NotNull are treated as nullable (or not-null, respectively) and used during the analysis to check nullability contracts, e.g. report possible NullPointerException errors.

More complex contracts can be defined using @Contract annotation, for example:

@Contract("_, null -> null") — method returns null if its second argument is null
@Contract("_, null -> null; _, !null -> !null") — method returns null if its second argument is null and not-null otherwise
@Contract("true -> fail") — a typical assertFalse method which throws an exception if true is passed to it

The inspection can be configured to use custom @Nullable
@NotNull annotations (by default the ones from annotations.jar will be used)

What happen is that Enums already have a non override able valueOf method. This means that you are in fact calling your own defined valueOf, but the IDE assumes that it's the static valueOf. So in order to fix your issue you have to rename your method to something like